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esp_timer.rst

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High Resolution Timer

Overview

Although FreeRTOS provides software timers, these timers have a few limitations:

  • Maximum resolution is equal to RTOS tick period
  • Timer callbacks are dispatched from a low-priority task

Hardware timers are free from both of the limitations, but often they are less convenient to use. For example, application components may need timer events to fire at certain times in the future, but the hardware timer only contains one "compare" value used for interrupt generation. This means that some facility needs to be built on top of the hardware timer to manage the list of pending events can dispatch the callbacks for these events as corresponding hardware interrupts happen.

An interrupt level of the handler depends on the CONFIG_ESP_TIMER_INTERRUPT_LEVEL option. It allows to set this: 1, 2 or 3 level (by default 1). Raising the level, the interrupt handler can reduce the timer processing delay.

esp_timer set of APIs provides one-shot and periodic timers, microsecond time resolution, and 64-bit range.

Internally, esp_timer uses a 64-bit hardware timer, where the implemention depends on CONFIG_ESP_TIMER_IMPL. Available options are:

esp32
  • LAC timer
esp32
  • (legacy) FRC2 timer
not esp32
  • SYSTIMER

esp32

Note

The FRC2 is a legacy option for ESP32 until v4.2, a 32-bit hardware timer was used. Starting at v4.2, use the new LAC timer option instead, it has a simpler implementation, and has smaller run time overhead because software handling of timer overflow is not needed.

Timer callbacks can dispatched by two methods:

  • ESP_TIMER_TASK
  • ESP_TIMER_ISR. Available only if CONFIG_ESP_TIMER_SUPPORTS_ISR_DISPATCH_METHOD is enabled (by default disabled).

ESP_TIMER_TASK. Timer callbacks are dispatched from a high-priority esp_timer task. Because all the callbacks are dispatched from the same task, it is recommended to only do the minimal possible amount of work from the callback itself, posting an event to a lower priority task using a queue instead.

If other tasks with priority higher than esp_timer are running, callback dispatching will be delayed until esp_timer task has a chance to run. For example, this will happen if a SPI Flash operation is in progress.

ESP_TIMER_ISR. Timer callbacks are dispatched directly from the timer interrupt handler. This method is useful for some simple callbacks which aim for lower latency.

Creating and starting a timer, and dispatching the callback takes some time. Therefore there is a lower limit to the timeout value of one-shot esp_timer. If :cppesp_timer_start_once is called with a timeout value less than 20us, the callback will be dispatched only after approximately 20us.

Periodic esp_timer also imposes a 50us restriction on the minimal timer period. Periodic software timers with period of less than 50us are not practical since they would consume most of the CPU time. Consider using dedicated hardware peripherals or DMA features if you find that a timer with small period is required.

Using esp_timer APIs

Single timer is represented by :cppesp_timer_handle_t type. Timer has a callback function associated with it. This callback function is called from the esp_timer task each time the timer elapses.

  • To create a timer, call :cppesp_timer_create.
  • To delete the timer when it is no longer needed, call :cppesp_timer_delete.

The timer can be started in one-shot mode or in periodic mode.

  • To start the timer in one-shot mode, call :cppesp_timer_start_once, passing the time interval after which the callback should be called. When the callback gets called, the timer is considered to be stopped.
  • To start the timer in periodic mode, call :cppesp_timer_start_periodic, passing the period with which the callback should be called. The timer keeps running until :cppesp_timer_stop is called.

Note that the timer must not be running when :cppesp_timer_start_once or :cppesp_timer_start_periodic is called. To restart a running timer, call :cppesp_timer_stop first, then call one of the start functions.

Callback functions

Timer callbacks which are processed by ESP_TIMER_ISR method should not call the context switch call - portYIELD_FROM_ISR(), instead of this you should use the :cppesp_timer_isr_dispatch_need_yield function. The context switch will be done after all ISR dispatch timers have been processed, if required by the system.

esp_timer during the light sleep

During light sleep, the esp_timer counter stops and no callback functions are called. Instead, the time is counted by the RTC counter. Upon waking up, the system gets the difference between the counters and calls a function that advances the esp_timer counter. Since the counter has been advanced, the system starts calling callbacks that were not called during sleep. The number of callbacks depends on the duration of the sleep and the period of the timers. It can lead to overflow of some queues. This only applies to periodic timers, one-shot timers will be called once.

This behavior can be changed by calling :cppesp_timer_stop before sleeping. In some cases, this can be inconvenient, and instead of the stop function, you can use the skip_unhandled_events option during :cppesp_timer_create. When the skip_unhandled_events is true, if a periodic timer expires one or more times during light sleep then only one callback is called on wake.

Using the skip_unhandled_events option with automatic light sleep (see Power Management APIs <power_management>) helps to reduce the consumption of the system when it is in light sleep. The duration of light sleep is also determined by esp_timers. Timers with skip_unhandled_events option will not wake up the system.

Handling callbacks

esp_timer is designed to achieve a high-resolution low latency timer and the ability to handle delayed events. If the timer is late then the callback will be called as soon as possible, it will not be lost. In the worst case, when the timer has not been processed for more than one period (for periodic timers), in this case the callbacks will be called one after the other without waiting for the set period. This can be bad for some applications, and the skip_unhandled_events option was introduced to eliminate this behavior. If skip_unhandled_events is set then a periodic timer that has expired multiple times without being able to call the callback will still result in only one callback event once processing is possible.

Obtaining Current Time

esp_timer also provides a convenience function to obtain the time passed since start-up, with microsecond precision: :cppesp_timer_get_time. This function returns the number of microseconds since esp_timer was initialized, which usually happens shortly before app_main function is called.

Unlike gettimeofday function, values returned by :cppesp_timer_get_time:

  • Start from zero after the chip wakes up from deep sleep
  • Do not have timezone or DST adjustments applied

Application Example

The following example illustrates usage of esp_timer APIs: system/esp_timer.

API Reference

inc/esp_timer.inc